Existing simple combining techniques use Chase combining weighted by SNR (signal-to-noise ratio) to realize HARQ combining. Here, SNR is an average value over a period of time (for example, a data frame).
There are two cases in relatively low SNR environment: 1) variance of SNR may be relatively high over the same period of time; 2) variance of SNR may be relatively low over the same period of time. The first case indicates that time selective fading of the signal is very serious. The effect of fast fading in time domain may be neglected and the performance of HARQ combining at the receiver side may be reduced if only Chase combining weighted by SNR is simply adopted to realize HARQ combing.